DESCRIPTION: (Applicant's Description) Cancer diagnoses have been continuously recorded at the Danish Cancer Registry for the whole of Denmark over five decades since 1943, including more than 25,000 diagnoses of leukemia. Using this nationwide essentially prospective resource, the investigators propose to: (1) Describe trends in leukemia incidence for Denmark from 1943 to 1993; (2) Link cancer and population registry information via person number to identify parent-offspring pairs diagnosed with leukemia; (3) Construct kindreds suitable for segregation and genetic linkage studies of leukemia based on affected parent-offspring pairs by contact with subjects and additional registry searches. Note that linkage of the Danish cancer and twin registries has already been accomplished. Hence, kindreds can also be identified based on twin pairs concordant for leukemia; and (4) Investigate anticipation in familial leukemia using the parent-offspring pairs according to type of leukemia, accounting for the older average age of the parental generation. The proposed study efficiently employs existing epidemiologic resources to better define the heritability of leukemia. A finding of anticipation would support Horwitz et al. (1966) who suggest that dynamic mutation of unstable DNA sequence repeats may be a common mechanism for hematopoietic malignancy -- with implications for the role of somatic mutation in the more frequent sporadic cases.